In recent years, there has been a trend for motorized equipment requiring a large current discharge, including hybrid-type electric vehicles and electric power tools, to rapidly expand. As a power source of these equipment, alkaline secondary batteries such as nickel-metal hydride batteries and nickel-cadmium batteries are widely used. In the market, particularly, an inexpensive alkaline secondary battery having a high capacity is desired, and cost reduction of the alkaline secondary battery is also desired intensely.
Therefore, use of an iron substrate as an electrode in place of a substrate made of expensive pure nickel used as a negative electrode is proposed in order to reduce the material cost of an alkaline secondary battery. However, iron is easily dissolved in an electrolyte solution. Further, it is known that the dissolved iron is deposited at a positive electrode to cause deterioration of charge efficiency. Since the amount of elution of iron increases with time, a longer storage period after production of a battery more frequently causes lowering of the battery capacity.
Therefore, an iron substrate having a conductive protecting layer formed on the surface thereof is usually used. However, even if the conductive protecting layer is formed on the surface of the iron substrate, the elution of iron may not be adequately suppressed in the case of a thin protecting layer when defects are present on the conductive protecting layer. Therefore, usually, in the iron substrate having a conductive protecting layer formed thereon, the thickness of the conductive protecting layer is set to more than 3 μm. For example, in Patent Document 1, the thickness of the conductive protecting layer is set to 4 μm or more. In Patent Document 1, thick nickel plating is employed as a conductive protecting layer to be used in the positive electrode in order to suppress iron contamination in the positive electrode.
Also, iron is used as a substrate for an outer case as with the negative electrode. Therefore, iron is dissolved and deposited at a positive electrode as with the negative electrode to cause deterioration of charge efficiency. Therefore, usually, a conductive protecting layer having a thickness smaller than that of the negative electrode but larger than 0.5 μm is formed in order to suppress the elution of iron also for the outer case.
When the thickness of the conductive protecting layer is increased in the negative electrode or the outer case in this way, it is possible to suppress the elution of iron, but the cost increases as the thickness increases. Therefore, the advantage of use of the iron substrate as an alternative to an expensive nickel substrate is not adequately utilized. Accordingly, increase in the thickness of the conductive protecting layer is not practical.
In view of the above-mentioned situation, there is a desire for a negative electrode, which can suppress the elution of iron and can suppress the lowering of charge efficiency even under conditions in which the elution of iron into an electrolyte solution tends to occur, including a case where the thickness of a conductive protecting layer is small or where the conductive protecting layer has defects, or under conditions in which the surface of the iron substrate is exposed.